Various mobile electronic gears (cell phones, mobile information terminals PDA, note-type personal computers, DVD players, CD players, MD players, digital cameras, digital video cameras, etc.) comprise pluralities of DC/DC converters as power conversion apparatuses for converting the voltage of contained cells to operation voltage. In note-type personal computers, for instance, DC/DC converters are arranged near digital signal processors (DSP), micro processing units (MPU), etc.
As one example of DC/DC converters, FIG. 6 shows a step-down DC-DC converter comprising an input capacitor Cin, an output capacitor Cout, an output inductor Lout, and an integrated circuit semiconductor IC including a switching device and a control circuit as discrete circuits on a printed circuit board. By operating the switching device based on a control signal from the control circuit, output voltage Vout expressed by Vout=Ton/(Ton+Toff)×Vin, wherein Ton is a time period in which the switching device is turned on, and Toff is a time period in which the switching device is turned off, is obtained from DC input voltage Vin. Even with the variation of the input voltage Vin, stable output voltage Vout can be obtained by adjusting a Ton/Toff ratio. An LC circuit comprising an output inductor Lout storing and discharging current energy, and an output capacitor Cout storing and discharging voltage energy acts as a filter circuit (smoothing circuit) for outputting DC voltage.
An output inductor Lout widely used at present, as shown in FIGS. 8 and 9, comprises a conductor wire 230 wound around a magnetic core 220. Used for the magnetic core 220 is high-resistance ferrite such as Ni—Zn ferrite, Ni—Cu—Zn ferrite, etc., so that a conductor wire can be wound directly around it.
The reduction of the operation voltage of LSI (large scale integration) constituting DSP and MPU has recently been accelerated to expand the usable time period of cells. Operation voltage has been lowered to 2.5 V, and further to 1.8 V, for high-speed-operation parts such as MPU and DSP. Because of such decrease of operation voltage, the voltage margin of LSI is reduced relative to the variation (ripple) of the output voltage of DC/DC converters, so that LSI is more influenced by noise. To cope with this, the switching frequencies of DC/DC converters have been increased from conventional 500 kHz to 1 MHz or more to suppress ripple.
Higher switching frequencies reduce inductance required for an output inductor Lout, enabling the size reduction of the inductor and a power supply circuit. However, higher switching frequencies contribute to the reduction of conversion efficiency due to loss generated in switching devices and inductors. Although power loss by inductors is caused predominantly by the DC resistance of conductor lines and output current at low frequencies, AC resistance (AC resistance of conductor lines and core loss of ferrite) is not negligible at high frequencies. Accordingly, to operate DC/DC converters efficiently at high frequencies exceeding 1 MHz, the core loss of ferrite constituting inductors should be reduced. The core loss of ferrite is determined by hysteresis loss, eddy current loss and residual loss. It is known that these losses depend on the magnetic properties (coercivity, saturation magnetization, magnetic domain wall resonance, etc.), crystal grain size, resistivity, etc. of ferrite.
JP 2002-289421 A discloses Ni—Cu—Zn ferrite comprising 46-50% by mol of Fe2O3, 2-13% by mol of CuO, 24-30.5% by mol of ZnO, and 3.5% by mol or less of Mn2O3, the balance being NiO, which has a high saturation magnetic flux density and low loss. The addition of Mn2O3 provides this ferrite with reduced loss at a magnetic flux density of 150 mT and a frequency of 50 kHz. However, this reference does not propose any measures to achieve loss reduction at high frequencies, and to cope with the deterioration of characteristics under stress and the change of characteristics with temperature. Although this reference describes that inevitable impurities including typical metal elements such as B, C, Al, Si, P, S, Cl, As, Se, Br, Te, I, Li, Na, Mg, Al, K, Ca, Ga, Ge, Sr, Cd, In, Sn, Sb, Ba, Tl, Pb, Bi, etc., and transition metal elements such as Sc, Ti, V, Cr, Co, Y, Zr, Nb, Mo, Pd, Ag, Hf, Ta, W, etc. may be contained, it does not discuss the addition of both Mn and Sn as sub-components at all.
Inductors are also required to have stability under stress (little variation of inductance and less increase in loss under stress). Such stress includes stress caused by the difference in a linear thermal expansion coefficient between an inductor and a printed circuit board, stress caused by the deformation of a printed circuit board, stress caused by the curing of a molding resin when an inductor is sealed with a resin, stress caused by shrinkage difference when internal conductors and ferrite are simultaneously sintered to produce a laminated inductor, stress caused by plating external terminals, etc. Also, because DC/DC converters are exposed to heat generated by integrated circuit semiconductors IC, etc., inductors used therein are required to exhibit stable characteristics at use temperatures; little variation of inductance with temperature.
As ferrite having improved stability and temperature characteristics under stress, JP 05-326243 A discloses Ni—Cu—Zn ferrite comprising 100% by mass of main components comprising 46.5-49.5% by mol of Fe2O3, 5.0-12.0% by mol of CuO, and 2.0-30.0% by mol of ZnO, the balance being NiO, and sub-components comprising 0.05-0.6% by mass of Co3O4, 0.5-2% by mass of Bi2O3, and 0.1-2% by mass in total of SiO2 and SnO2. However, this Ni—Cu—Zn ferrite does not contain Mn (Mn2O3) though it contains SnO2. Accordingly, loss reduction at high frequencies, and the improvement of stability and temperature characteristics under stress by the addition of both Mn and Sn are not achieved. Also, because it contains Bi2O3 having a melting point of 820° C. in as large an amount as 0.5-2% by mass, crystal growth is accelerated, resulting in a crystal structure having an average crystal grain size of 5 μm or more, and thus large core loss at high frequencies.